Hydraulic cylinders and hydraulic lift jacks normally include a bleed port in the cylinder wall adjacent the rod end of the cylinder such that as the piston head approaches the rod end of the cylinder, the bleed port will be uncovered and hydraulic fluid will be vented to the reservoir. The bleed port functions to limit the stroke of the piston in the cylinder and prevents generation of excessive hydraulic pressure in the cylinder. Examples of prior art cylinders having bleed ports are illustrated in the U.S. Pat. No. 3,685,797, issued Aug. 22, 1972, Orr and in the U.S. Pat. No. 4,077,608, issued Mar. 7, 1978, Nehrig. Attention is also directed to the U.S. Pat. No. 2,456,105, issued Dec. 14, 1948, Berg, the U.S. Pat. No. 3,818,805, issued June 25, 1974, Johansson; the U.S. Pat. No. 3,925,985, issued Dec. 16, 1975, Peterson and Offenlegungsschrift No. 1 954 936.
As set forth in the Orr patent, in the manufacture of hydraulic jacks, the seal at the head of the piston is economically manufactured from a plastic material. In the prior art arrangements, the groove or bleed port may cause scoring or wear of the plastic seal and thereby reduce the wear life of the seal and the cycle life of the cylinder.
In order to reduce the wear of the piston head seal, it is preferred that the diameter of the bleed port be as small as possible. In prior constructions such small diameter bleed ports have been formed by drilling a first hole through a substantial portion of the thickness of the cylinder wall from the outside of the cylinder wall radially inwardly. A much smaller diameter hole or bore is then drilled through the remaining material of the cylinder wall. It has been found that this small bore can have a diameter of 0.010 to 0.012 inches. Bores of this size cause less wear of the piston head seal and facilitate longer wear life of the seal.
The thickness of the cylinder wall may commonly be in the range of 0.090 to 0.250 inches. It is not economically feasible to drill a hole of 0.010 to 0.012 inches diameter through the entire thickness of this cylinder wall. Drills having such small diameters are very fragile and normally have a shank length insufficient to drill through a cylinder wall of such a thickness. Machining operations using such a small drill are also time consuming, and require highly skilled machinists.
Another manufacturing problem encountered with prior art methods for forming very small diameter bleed ports in the inner surface of the cylinder wall is that the internal surface of the cylinder surrounding a drilled bleed port must be honed or polished. The drilling operation produces a burr on the interior surface of the cylinder, and this burr may result in substantial scoring or wear of the seal surrounding the piston head. The honing operation must be done manually and may also force metal filings or material back into the bleed port thereby plugging or restricting the bleed port. Because the bleed port diameters are so small, they cannot be effectively visually inspected. In some cases a plugged bleed port may not be discovered until there has been a failure of the hydraulic cylinder.